Slippage mechanism of pediatric spondylolysis - Biomechanical study using immature calf spines

Study Design. This study analyzed the skeletal-age-dependent strength of the lumbar growth plate to resist anterior shearing forces using the MTS system in the immature calf spine with pars defects. Objective. To clarify the pathomechanism of the skeletal-age-dependent incidence of slippage in pediatric patients with pars defects by comparing the strength of the lumbar growth plate among three skeletal age groups. Summary of Background Data. Isthmic spondylolisthesis occurs and progresses more frequently during the growth period, whereas it is rare afterward. However, little evidence has been demonstrated to elucidate the etiology. Methods. For this study, 15 lumbar functional spine units were divided into three groups according to their skeletal ages. Five were from neonates (Group 1), five from calves approximately 2 months old (Group 2), and five from calves about 24 months old (Group 3). An anterior shearing force was applied to each specimen until failure, after bilateral pars defects were created. Failure load (newtons) and displacement at failure (millimeters) were calculated from the load-displacement curve. The site of failure was confirmed by plain radiograph. Results. All 15 functional spine units failed at the growth plate. The failure load was 242.79 +/- 46.05 N in Group 1, 986.40 +/- 124.16 N in Group 2, and 2024.54 +/- 245.53 N in Group 3. Statistically significant differences were found among the three groups (P < 0.05). The displacement at failure was 7.52 +/- 1.84 mm in Group 1, 11.10 +/- 2.30 mm in Group 2, and 8.15 +/- 2.66 mm in Group 3. There were no significant differences among the groups. Conclusions. The results indicate that the strength of the growth plate, the weakest link in this model, against anterior shearing forces depends on the skeletal maturity, and that the biomechanical weakness of the growth plate plays an important role in the slippage mechanism.